1. Field of the Invention
The present invention relates generally to an air/fuel mixture ratio learning control system for an internal combustion engine which uses a mixed fuel, such as, a gasoline-alcohol mixed fuel. The fuel may contain alcohol concentration of 0 to 100%. More specifically, the present invention relates to a learning control system for controlling an air/fuel ratio in a fuel injection internal combustion engine using a mixed fuel, wherein a fuel injection amount is precisely controlled at a desired value both in a FEEDBACK or CLOSED LOOP mode air/fuel ratio control and in an OPEN LOOP mode air/fuel ratio control, using a learnt correction coefficient which is set and updated per one of preselected engine driving ranges as well as per one of preselected alcohol concentrations contained in the mixed fuel.
2. Description of the Background Art
Learning control systems for controlling a mixture ratio of air and pure gasoline fuel have been proposed, such as disclosed in Japanese Patent First Publication No. 59-203828.
In this publication, a fuel injection amount T.sub.i is derived based on the following equation (1): EQU T.sub.i =T.sub.p .times.C.sub.oef .times.K.sub.LAMBDA .times.K.sub.LRC +T.sub.s ( 1)
where, T.sub.p is a basic fuel injection amount derived based on an engine speed and an engine load (an intake air flow rate, for example), C.sub.oef is a correction coefficient derived based on various engine operation parameters, such as an engine coolant temperature, K.sub.LAMBDA is a FEEDBACK air/fuel ratio dependent correction coefficient derived based on an oxygen concentration indicative signal from an oxygen sensor arranged in an exhaust system during the FEEDBACK mode control and composed of a proportional (P) component and an integral (I) component so as to perform the proportional-plus-integral control (PI control) of the fuel injection amount T.sub.i, and K.sub.LRC is a learnt correction coefficient derived based on the FEEDBACK correction coefficient K.sub.LAMBDA. The learnt correction coefficient K.sub.LRC is cyclically derived and updated with respect to each of mutually distinct various engine driving ranges identified by the engine speed and the basic fuel injection amount T.sub.p. The equation (1) further includes T.sub.B which is a correction amount derived based on a battery voltage.
The FEEDBACK correction coefficient K.sub.LAMBDA is used to control the fuel injection amount T.sub.i for maintaining the air/fuel ratio of the air/fuel mixture at a target value, such as, a stoichiometric value in the FEEDBACK mode control which is performed in a predetermined stable engine driving condition. If it is possible to maintain the fuel injection amount T.sub.i at the stoichiometric value with the FEEDBACK correction coefficient being at a value of 1, then no FEEDBACK control is necessary in theory. However, in practice, due to tolerances in fuel injection valves, air-flow meters, pressure regulators and other engine components and further due to variations in functional characteristics of those components with the elapse of time which cause deviation or error between the arithmetically derived fuel injection amount and the practically injected fuel amount, the FEEDBACK control should be necessary for compensating such deviation or error.
However, during the air/fuel ratio being controlled in the OPEN LOOP mode, the above-noted deviation or error can not be compensated so that the air/fuel ratio of the air/fuel mixture is controlled with such deviation or error being included, resulting in the unreliable control of the fuel injection amount T.sub.i. Further, after the air/fuel ratio control is shifted from the OPEN LOOP mode to the FEEDBACK mode, a considerable delay is caused before the air/fuel ratio reaches the stoichiometric value due to the PI control of the fuel injection amount T.sub.i on the basis of the FEEDBACK correction coefficient K.sub.LAMBDA which moderately modifies the fuel injection amount.
Accordingly, the system in this publication further uses the learnt correction coefficient K.sub.LRC which compensates the above-noted deviation or error bothe in the OPEN LOOP mode control and the FEEDBACK mode control. The learnt correction coefficient K.sub.LRC is stored for each of the mutually distinct various engine driving ranges and is updated on the basis of the instantaneous FEEDBACK correction coefficient K.sub.LAMBDA in a predetermined stable engine driving condition during the FEEDBACK mode control so as to adjust the FEEDBACK correction coefficient K.sub.LAMBDA toward a value of 1. Accordingly, even in the OPEN LOOP mode control, the above-noted deviation or error is effectively compensated to derive the reliable fuel injection amount T.sub.i.
On the other hand, when a mixed fuel, such as, a gasoline/alcohol mixture fuel is used, a fuel injection amount T.sub.i may be derived on the basis of the following equation (2): EQU T.sub.i =T.sub.p .times.C.sub.oef .times.K.sub.ALC .times.K.sub.LAMBDA .times.K.sub.LRC +T.sub.s ( 2)
where, K.sub.ALC is an alcohol concentration dependent correction coefficient derived on the basis of an alcohol concentration indicative signal from an alcohol sensor which is disposed in a fuel supply line.
Since the stoichiometric value of the air/fuel ratio for the pure gasoline fuel is 14.7 while that for the fuel containing, such as, a methanol concentration of 100% is 6.5, the alcohol concentration dependent correction coefficient K.sub.ALC largely varies dependent on the alcohol concentration derived through the alcohol sensor. Accordingly, the learnt correction coefficient K.sub.LRC should also compensate the above-noted deviation or error which is variable dependent on the alcohol concentration in addition to the engine driving ranges specified by the engine speed and the basic fuel injection amount T.sub.p. Further, the learnt correction coefficient should also compensate the above-noted deviation or error which is caused by tolerance in the alcohol sensor and the variation in functional characteristics of the alcohol sensor with the elapse of time.
However, since the learnt correction coefficient K.sub.LRC is set only with respect to each of the engine driving ranges identified by the engine speed and the basic fuel injection amount T.sub.p in the proposed air/fuel ratio control system, such as, disclosed in the above-noted Japanese Patent First Publication No. 59-203828, a considerable delay is caused before the air/fuel ratio reaches the stoichiometric value in the FEEDBACK mode control when the alcohol concentration contained in the fuel is largely varied. Further, the reliable fuel injection amount T.sub.i can not be derived in the OPEN LOOP mode control until the learning has been fully advanced for the fuel currently used.